Asphalt producers and paving contractors face a persistent challenge: how to increase recycled asphalt pavement (RAP) content without compromising pavement performance. State specifications typically cap RAP at 25 to 30 percent binder replacement for surface courses, largely because aged, oxidized RAP binder creates a brittle mix prone to cracking. A recent field trial by E&B Paving in Indianapolis, working with researchers from Iowa State University and Colorbiotics, demonstrated that a soybean oil-derived rejuvenator called Invigorate can push RAP content to 40 percent while maintaining workability and durability. For contractors seeking to maximize material reuse and reduce costs, understanding the approach behind this trial offers practical lessons in advanced mix design. Those involved in similar projects may also benefit from reviewing Paving Utility Cuts Paths and Parking Lots Best for broader small-commercial paving strategies.
The Economics and Chemistry of Higher RAP Mixes
The push for increased RAP usage is driven by both economics and sustainability. According to the National Asphalt Pavement Association (NAPA), using RAP saves American taxpayers and consumers more than $2.8 billion annually. Every ton of RAP used reduces the demand for virgin aggregates and liquid asphalt binder, lowering project costs and conserving resources.
Why RAP Content Is Capped
Despite these benefits, increasing RAP content introduces technical limitations. RAP binder is aged, oxidized, and brittle. Used at high percentages without treatment, it produces a stiff mix that is difficult to work with and prone to cracking under traffic and thermal cycling. Elizabeth Pastuszka, executive director of the Asphalt Pavement Association of Indiana (APAI), explains that in Indiana mixes are limited to a maximum of 25 percent binder replacement. This creates a surplus problem: 100 percent of the existing road surface can be milled and returned to the plant, but only a fraction can be put back into new mixes. The result is growing RAP stockpiles at many plants and a need for technologies that enable higher utilization rates.
Understanding Binder Chemistry
Asphalt binder consists of two primary components:
- Asphaltenes – high-molecular-weight compounds that provide structural integrity and resistance to rutting
- Maltenes – oily and resinous components that give the binder its liquid properties, enabling workability and aggregate coating
As binder ages, the ratio of asphaltenes to maltenes increases. Asphaltene molecules aggregate into larger clusters, making the binder stiff and brittle. Traditional rejuvenators address this by adding maltenes back into the blend, diluting the aged binder with softer material. While somewhat effective, this approach has limits: adding enough maltenes to fully restore workability at high RAP contents can soften the binder too much, compromising rut resistance.
Invigorate: A Chemically Active Rejuvenator
Researchers at Iowa State University (ISU), in collaboration with Ames-based Colorbiotics (a brand of the MBCC Group), developed a rejuvenator that works differently from traditional softening agents. Rather than simply adding maltenes, Invigorate triggers a chemical reaction that breaks down asphaltene aggregation at the molecular level, reversing the aging process rather than compensating for it.
How Invigorate Works
The soybean oil-derived formulation penetrates the aged binder and disrupts the molecular forces holding asphaltene clusters together. This restores the balance between asphaltenes and maltenes without over-softening the binder. Ken Staebell, director of new technology development at Colorbiotics, notes that the rejuvenator is highly compatible with asphalt binder, so blending and homogeneity are achieved easily. Key advantages include:
- Higher RAP content without proportional loss of low-temperature cracking resistance
- Improved workability during placement and compaction
- Compatibility with standard plant equipment and blending procedures
- Reduced dependency on virgin binder, lowering material costs
Application Methods
Invigorate can be incorporated through multiple methods. It can be blended directly into the liquid asphalt supply at the plant, added at the mixer, or introduced via a separate metering system. The recommended addition rate depends on RAP content and the target performance grade. For the E&B Paving trial, the rejuvenator was added at 4.2 percent by volume of total binder.
The Indianapolis Parking Lot Field Trial
E&B Paving, a contractor based in central Indiana, partnered with Iowa State University and Colorbiotics to test Invigorate on a commercial parking lot in Indianapolis. The project evaluated whether a 40 percent RAP surface mix could meet the same performance standards as conventional lower-RAP mixes under real-world conditions.
Mix Design and Production
The trial involved approximately 300 tons of a 9.5 mm surface mix with 40 percent RAP using Invigorate, plus a second experimental biopolymer mix for comparison. The target gradation and asphalt content were developed through standard procedures to achieve a balanced mix meeting Superpave specifications.
| Parameter | Value |
|---|---|
| Mix type | 9.5 mm surface course |
| RAP content | 40% |
| Virgin binder grade | PG 64-22 |
| Invigorate dosage | 4.2% by volume of total binder |
| Production temperature | 285-300 °F |
| Plant type | Counterflow drum plant |
| Production location | E&B Stoney Creek plant, Noblesville, IN |
| Haul distance | Approximately 30 minutes |
| Total tonnage | ~300 tons per mix |
Site Conditions and Paving Challenges
The parking lot surrounding a multi-tenant office building presented challenges typical of tight commercial paving:
- Short pulls – the paver stopped and started frequently due to the U-shaped lot layout
- Varying paving widths – requiring constant screed adjustments
- Inverted drains – requiring careful hand work around utilities
- Limited working room – bounded by the building and adjacent parking areas
Prior to paving, the top 1.5 inches of the existing surface were milled and recycled. The crew placed a 1.5-inch overlay of three different mix designs, achieving target rolling patterns for compaction.
Crew Observations on Workability
Stephen Perkey, paving foreman in his 12th season with E&B Paving, noted that tight lots are among the most demanding paving applications. Despite the 40 percent RAP content, the Invigorate mix handled remarkably well. Perkey observed that high-RAP mixes often turn stiff and lose temperature rapidly, becoming difficult to work before achieving compaction. With the Invigorate mix, the material stayed workable throughout placement. The crew achieved densities exceeding 95 percent using their standard rolling pattern.
Performance Testing and Quality Verification
Laboratory testing confirmed that the 40 percent RAP mix met or exceeded standard performance benchmarks for both rutting and cracking resistance. The Hamburg Wheel Tracking Test showed minimal rut depth and good moisture resistance, confirming that the rejuvenator did not over-soften the binder. Two cracking tests were used: the CT Index, measuring cracking tolerance at intermediate temperatures, and the Illinois Flexibility Index (I-FIT), evaluating flexibility of compacted specimens. The Invigorate mix achieved strong results in both, with Pastuszka noting the CT Index values were encouraging and the I-FIT results more than acceptable.
| Property | Test Method | Result |
|---|---|---|
| Rutting resistance | Hamburg Wheel Tracking | Passed – minimal rut depth |
| Cracking tolerance | CT Index | Strong values achieved |
| Flexibility | I-FIT | Acceptable performance |
| Field density | In-place compaction | >95% achieved |
| Workability | Crew observation | Good – no premature stiffening |
APAI’s Pauley, who observed the paving operation, commented that the finished surface looked no different from standard mixes. This visual assessment, combined with the laboratory and field data, supports the conclusion that chemically active rejuvenators can enable higher RAP content without compromising quality.
Practical Implications for Producers and Contractors
The E&B Paving trial demonstrates that 40 percent RAP content is achievable in surface course mixes with the right rejuvenation chemistry. For producers managing growing RAP stockpiles, this opens the door to higher utilization rates and reduced virgin material consumption. For contractors, the improved workability means less risk of compaction issues.
Key Considerations for Adoption
Contractors and producers evaluating high-RAP mixes should consider:
- RAP variability – aged binder properties vary by source; mix designs should be verified for each stockpile
- Rejuvenator dosage – optimal addition rate depends on target RAP content and binder performance grade
- Plant integration – ensure the rejuvenator can be metered accurately into the mix
- Quality control – implement Hamburg and I-FIT testing to verify specification compliance
- Agency approval – work with local DOT representatives for RAP content above standard caps
Aggregate quality also plays a role in pavement performance. Producers should ensure that sand fractions meet specification limits for cleanliness. For guidance, see Silt Content Costruction Sand, which covers the impact of excessive fines on mix durability.
For mix designers aiming to push RAP content further, understanding how rejuvenators interact with aged binder is essential. The ISU research shows that chemically active rejuvenators restore aged binder properties more effectively than traditional softening agents. See Reviving Recycled Asphalt How Additives Enable Higher Rap for more on additive-based approaches to RAP mix design.
Moisture and material testing are also relevant for quality assurance. When compaction testing involves moisture-density relationships, the torsion balance method provides reliable results for water content determination. See Determination of Water Content in Soil By Torsion for more information.
Future Outlook
As state DOTs and federal agencies push toward greater sustainability in pavement construction, the demand for high-RAP technologies will grow. Chemically active rejuvenators represent a significant advancement, offering a path to higher recycling rates without compromising pavement performance. The E&B Paving trial provides real-world evidence that 40 percent RAP content is not only possible but practical for commercial paving. With continued research and field validation, the 25 percent RAP cap that many producers currently work within may become a thing of the past.